Add a very basic race description for the dunefolk

Eventually this will be expanded into a more proper description

data/core/units.cfg

@@ -203,7 +203,7 @@ The eastern provinces of Wesnoth, known as the Clan Homelands, have a geography
         male_name= _"race^Dunefolk Human"
         female_name= _"race+female^Dunefolk Human"
         plural_name= _"race+plural^Dunefolk"
-        description= _"This race does not have a description yet."
+        description= _"The Dunefolk are a human culture inhabiting the dry deserts in the southern region of the Great Continent."
         num_traits=2
         markov_chain_size=2
         # wmlscope: start ignoring